Re: kvm guest loops_per_jiffy miscalibration under host load

[Dor Laor <dor.laor@qumranet.com>]

Marcelo Tosatti wrote:
> On Tue, Jul 22, 2008 at 10:22:00AM +0200, Jan Kiszka wrote:
>   
>>> The in-kernel PIT rearms relative to host clock, so the frequency is
>>> more reliable (next_expiration = prev_expiration + count).
>>>       
>> The same happens under plain QEMU:
>>
>> static void pit_irq_timer_update(PITChannelState *s, int64_t current_time);
>>
>> static void pit_irq_timer(void *opaque)
>> {
>>     PITChannelState *s = opaque;
>>
>>     pit_irq_timer_update(s, s->next_transition_time);
>> }
>>     
>
> True. I misread "current_time".
>
>   
>> To my experience QEMU's PIT is suffering from lost ticks under load
>> (when some delay gets larger than 2*period).
>>     
>
> Yes, with clock=pit on RHEL4 its quite noticeable. Even with -tdf. The
>   
Note that -tdf works only when you use userspace irqchip too, then it 
should work.
> in-kernel timer seems immune to that under the load I was testing.
>
>   
In the long run we should try to remove the in kernel pit. Currently it 
does handle pit
irq coalescing problem that leads to time drift.
The problem is that its not yet 100% production level, migration with it 
has some issues and
basically we should try not to duplicate userspace code unless there is 
no good reason (like performance).

There are floating patches by Glen Natapov for the pit and virtual rtc 
to prevent time drifts.
Hope they'll get accepted by qemu.
>> I recently played a bit with QEMU new icount feature. Than one tracks
>> the guest progress based on a virtual instruction pointer, derives the
>> QEMU's virtual clock from it, but also tries to keep that clock in sync
>> with the host by periodically adjusting its scaling factor (kind of
>> virtual CPU frequency tuning to keep the TSC in sync with real time).
>> Works quite nicely, but my feeling is that the adjustment is not 100%
>> stable yet.
>>
>> Maybe such pattern could be applied on kvm as well with tsc_vmexit -
>> tsc_vmentry serving as "guest progress counter" (instead of icount which
>> depends on QEMU's code translator).
>>     
>
> I see. Do you have patches around?
>
>
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